confused about __str__ vs. __repr__

[Carl Banks]

On Dec 18, 9:53 am, "Diez B. Roggisch" <de... at nospam.web.de> wrote:
> Neal Becker wrote:
> > Diez B. Roggisch wrote:
>
> >> Neal Becker wrote:
>
> >>> Tino Wildenhain wrote:
>
> >>>> Neal Becker wrote:
> >>>> ...
> >>>>>>> So if __str__ is "meant for human eyes", then why isn't print using
> >>>>>>> it!
> >>>>>> it is:
>
> >>>>>>  > print x
> >>>>>> str
>
> >>>>>> but dict just uses repr() for all its childs to print.
>
> >>>>>> T.
> >>>>> That makes no sense to me.  If I call 'print' on a container, why
> >>>>> wouldn't it recursively  print on the contained objects?  Since print
> >>>>> means call str, printing a container should recursively call str on
> >>>>> the objects.
>
> >>>> Every class is free on how to best implement __str__, you will find
> >>>> the same behavior on tuple and list as well.
>
> >>>> Maybe its discussable to change the implementation sensibly, best if
> >>>> you would come with a proposal? Perhaps pprint.pprint is a starting
> >>>> point?
>
> >>>> Regards
> >>>> Tino
>
> >>> First, I'd like to know if there is a rationale for the current design.
> >>> Am I correct in thinking this is a defect?
>
> >> I don't think so. First of all, there is no "generic" way of printing a
> >> collection. And the current implementation tries to give an overview what
> >> is contained in the collection, without trying to make it "fancy" - any
> >> such thing needed to be hand-coded anyway.
>
> >> Using repr for that is better suited, as for example string keys are
> >> printed with quotes around them - making clear what they are, and not
> >> irritating the user through potentially contained spaces or even things
> >> that look as if they are python objects.
>
> >> For example, if repr *wasn't* used,
>
> >> { "{foo=bar}" : "baz"}
>
> >> would be printed
>
> >> {{foo=bar} : baz}
>
> >> Which is *not* what the dictionary actually contains!
>
> >> The same goes for unicode-objects. They appear with their "funny"
> >> characters as \xXX-codes - instead of bailing out on you with
> >> unicode-errors.
>
> >> So, IMHO the current behavior is desired.
>
> > So if I want to overload something in my custom class, so that I get a
> > nice string whether it's printed directly, or as part of a container, what
> > is the recommendation?  Overload both __str__ and __repr__?
>
> Yep.

I would say no.

__repr__ should be specific--something that when evaled yields an
equal object, if possible--about what the object is.  Overloading
__repr__ to return an unspecific human-readable message is a mistake,
and will frustrate users who are debugging, logging, inspecting
objects at the interactive prompt.

Neal's problem seems to be with the behavior of dict's __str__ method,
which prints repr() of objects when he wants str().  That is intended
behavior, like it or not, so I would say the appropriate thing is not
use dict's default printed representation, and just write a custom
formating function for dicts.


Carl Banks